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Chiller undertemp

I'm having some faults on my glycol chiller that I've managed to figure out what the problem is but don't know the impact of the solution. I found out that my chiller is faulting on low glycol temperature on the supply line to the brewery. In looking at the glycol system designs in the refrigeration section it appears that I have a 2nd generation design for a system, that is, the glycol supply for the brewery exits directly from the evaporator. On this line (there is also a smaller line returning the glycol to the holding tank) there is a temperature probe with a Johnson Controls thermostat. This is what was tripping out the compressor. It was set to about 27 deg F and my glycol setpoint (regulating the temperature of the glycol in the holding tank) was set for -1.5 deg C. The system was functioning just fine except that the supply glycol setting was overriding the tank glycol setting and the compressor was being cycled based on it.

The questions; is this a problem at all or is this just the way the 2nd generation type systems work? What should the supply glycol thermostat be set at? What are the results of lowering the setting (other than risking icing the FV due to too low a temperature)? I guess I'd rather have the system regulating temperature through the digital single loop controller rather than the thermostat with a rotary dial that is maybe +/- 5 deg F in accuracy.

As an FYI this is a bastardized system built by Maneurop that their engineering department says they don't do any more and have no record of or drawings/manuals for.

I understand that your chiller system is being shut down by a thermostat that is sensing the temperature of the glycol as it leaves the chiller heat exchanger, and the setpoint on this t-stat is 27 F. You also have a T-stat that is sensing the temperature within the glycol tank, which is set at -1.5 C (29 F). The T-Stat in the glycol tank controls the chiller, the T-stat sensing the temperature leaving the evaporator is just a safety??

As long as you have the proper glycol concentration protecting from Freeze up at the lower temperatures, I don't see an issue lowering the setpoint on the safety thermostat. You want the freeze point of your solution to be 20 to 25 F below your glycol temperature, so if you lower this to 25 F, make sure your freeze point on your solution is approx. 0 F. (36% glycol / 64% Water)

Thanks for the reply Jim. You've got it right with the arrangement of the thermostats. The only reason I call it a 'safety' is because it's wired in series with the high pressure cutout and trips the compressor as well as lighting the 'fault' light on the control panel.

I don't know, maybe, because this system looks like it was put together in someone's basement, it originally just had the thermostat on the glycol supply line then someone added the temperature controller on the glycol holding tank as an afterthought.